Wait for exchange source to complete before verifying results (elastic#121603)

Closes elastic#118732

Author: dnhatn

x-pack/plugin/esql/compute/src/test/java/org/elasticsearch/compute/operator/exchange/ExchangeServiceTests.java

@@ -436,8 +436,8 @@ public void testExchangeSourceContinueOnFailure() {
             exchangeSink
         );
         assertThat(actualSeqNos, equalTo(expectedSeqNos));
+        safeGet(sourceCompletionFuture);
         assertThat(completedSinks.get() + failedSinks.get(), equalTo(totalSinks.get()));
-        sourceCompletionFuture.actionGet();
         if (failedRequests.get() > 0) {
             assertThat(failedSinks.get(), greaterThan(0));
         } else {